Meter Calibration: The key to accurate gas measurement

by Ed Bowles Flow Measurement Section Manager Southwest Research Institute

The Metering Research Facility (MRF) is one of just a few facilities in the world that can perform high-accuracy, gas flow meter calibrations under field-like operating conditions. The close-loop, re-circulating, gas flow calibration facilities at the MRF provide maximum controllability of pressure, temperature and flow rate, resulting in stable flow conditions over long testing times. A key feature of the MRF is its primary calibration systems based on gyroscopically-balanced weigh tanks, the most accurate means known for measuring gas mass flow rate. The MRF can correct flow meter bias errors as small as 0.05%.

The value of meter calibration.

A well run gas transmission pipeline network will typically have an overall system imbalance of less than ±0.5% (system imbalance is the difference between the quantity of gas metered into and out of a pipeline network). Large pipeline operators can potentially save millions of dollars annually if they can reduce their system imbalance by just one percentage point. To illustrate the potential for cost savings, let's look at the financial impact of bias error in a single flow meter installed in a typical interstate transmission pipeline. In this case, we will choose a 12-inch diameter ultrasonic gas flow meter. The industry's recommended practice, American Gas Association (A.G.A.) Report No. 9, specifies that meters of this type and size may have a maximum error of up to ±0.7% over the upper 90% of the rated flow range. The report further states that measurement bias errors attributed to piping configuration at the meter station must not exceed +0.3%. Let's assume that gas continuously flows through our meter at a rate comparable to the meter's mid-range. We'll also assume that the meter has a bias of minus 0.5% (i.e., it consistently under-registers the volumetric flow rate by 0.5%). This meter bias error is well within the allowable limits of Report No. 9, but would still result in a measurement error valued at over $2,500 per day or almost $1 million per year. Bias errors of this type can usually be eliminated by flow calibrating the meter at a cost of on the order of $5,000 to $50,000, depending on the method of calibration.

How should meters be calibrated?

A flow meter calibration should take into account operational effects (and, when practical, installation effects) that may adversely affect meter performance. Flow calibrations may be performed either in-situ, using a reference test flow meter plumbed in series at the meter station, or off site, at a flow calibration test facility like the MRF. For in-situ calibrations, consideration should be given at the design stage as to all valving and piping requirements necessary for a reference test flow meter to be installed on site. There are currently no gas industry standards or guidelines for in-situ or field meter proving. Off site flow meter calibrations are preferred, in most cases, because test conditions can be more precisely controlled than in the field.

To learn more about the MRF, please visit www.grimrf.org.

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